Field of the Invention
The invention relates to a chemically amplified resist for electron beam lithography.
In microelectronics, so-called chemically amplified resists are widely used for optical lithography and electron beam writing (electron beam lithography) (see Solid State Technology, Vol. 39 (1996), No. 7, pp. 164-173). The chemical amplification is used both with wet-developable single-layer resists and in entirely or partly dry-developable resists. The resists may operate on the principle of acid-catalyzed cleavage; polar but blocked chemical groups, such as carboxyl groups or phenolic hydroxyl groups, are deblocked by a photolytically generated acid, and the resist changes its polarity in the exposed regions. This change in polarity can be utilized for instance for developing the resist in an alkaline developer, or--in dry-developable resists--for selective silylation. Examples of blocking groups are tert-butylester and tert-butyloxycarbonyloxy groups.
European Patent Specification EP 0 492 256 B1 discloses photolithographic structure generation in which a dry-developable resist after exposure is subjected to a temperature treatment (post exposure bake=PEB), then silylated from the liquid phase, and after that anisotropically etched in oxygen plasma or dry-developed. Depending on the type of silylating solution, either positive or negative structures (images) are created. The resist in general comprises at least two solid components, that is, a base polymer and a photoactive acid generator. The base polymer contains carboxylic acid anhydride and tert-butylester partial structures; the acid generator is preferably an onium compound, such as diphenyliodonium and triphenylsulfonium trifluoromethanesulfonate. Such a resist is especially suitable for photostructuring in the submicron and sub-half-micron range with very steep edges.
In creating structures as described above--as in other resist systems that operate on the principle of acid-catalyzed cleavage--the so-called delay time effect has been found. This effect is expressed, for instance in electron beam lithography, in a deviation of the desired structural dimension from the structure actually obtained after the development, if there are delays (post exposure delay=PED) between the electron beam lithography and the temperature treatment (PEB). The longer this delay is, the greater are the structural deviations. In resists of the above type containing anhydride groups, for instance, the tolerable length of time is about 5 to 10 minutes. However, such a short time period is unacceptable for technical production reasons.
The delay time problems described are well known and are ascribed to basic contaminations in the air, which deactivate the photochemically generated strong acid during the delay. It has therefore already been proposed that this problem be solved by filtration of the air using activated charcoal (see Proceedings SPIE, Vol. 1466 (1991), pp. 2-12). However, this entails major investment cost.
Even by other measures, decisively influencing the delay time effect is not possible:
By adding organic amines to the resist, using triphenylsulfonium trifluoromethane sulfonate as sensitive photo acid generators (PAG), the resist can indeed be stabilized against lateral acid diffusion phenomena, but--because of this base addition--highly sensitive resists cannot be realized in this way (Journal of Photopolymer Science and Technology, Vol. 9 (1996), No. 4, pp. 677-684). PA1 By using photo acid generators with sterically small residues of strong acids, such as trifluoromethane sulfonate residues, the required exposure sensitivity can indeed be achieved, but the photo acid generators can diffuse very easily in the solid resist, and--because of the high acid strength--are very delay--sensitive to bases (Journal of Vacuum Science and Technology B, Vol. 12 (1994), No. 6, pp. 3857-3862). PA1 By adding mono- or dicarboxylic acids in the form of monomers or short-chained polymers, which are protected in the form of tert-butylesters, to the resist further dissolution inhibitors are introduced, and systems of high sensitivity are thereby obtained. However, structural dimensions of only 0.5 .mu.m can be realized in this way (Chemistry of Materials, Vol. 8 (1996), No. 2, pp. 376-381). PA1 a polymer with dissolution-inhibiting groups that can be cleaved with acid catalysis, PA1 a photo-reactive compound, which upon electron irradiation releases a sulfonic acid with a pK.sub.a value .ltoreq.2.5 (photo acid generator), PA1 an electron-beam-sensitive sensitizer enhancing the exposure sensitivity of the resist, and PA1 a solvent.
Indeed, the delay time behavior of resists can in principle be improved by the choice of a suitable photo acid generator, diphenyliodonium tosylate instead of the corresponding trifluoromethane sulfonate, but then the sensitivity drops (MNE-94- International Conference on Micro- and Nano-Engineering 94, Davos, Switzerland, Sep. 26-29, 1994, Paper P23). However, in electron beam lithography, this then requires longer writing times, which is undesired in production.